The Art of Electric Motor Repair Part 7: The Process

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Stator of a big electric motor. repair

Developing Your Motor Repair Specification 3

  1. Initial Winding Tests

Upon receipt by an electric motor shop, certain tests should be performed, as a minimum. The following steps are common to quality motor repair and also an explanation of the different methods used in repair.

The first test is an insulation resistance (ie: Megger) test, which measures leakage to ground. For motors rated under 600 Vac, 500 VDC is the acceptable limit, with a reading of 5 Megohms as the absolute lowest reading before proceeding with other tests. However, a reading below several hundred Megohms should indicate some type of problem. A reading of zero indicates a direct short to ground. The applied voltage and limits for medium voltage and DC motors will be covered in the test limits blog.

In many cases, a motor repair shop will test the phase to phase resistance of the electric motor with a milli-ohmmeter, or wheatstone bridge, then attempt to operate the electric motor before disassembly (assuming the motor passes these incoming tests). This is done to indicate what types of defects are within the motor. For electrical testing, the phase current is taken at full voltage, no load, and both noted for later use and compared to ensure that one phase is not drawing more current than the others.

Modern repair shops have started to perform motor circuit analysis tests on their motors prior to disassembly. This allows for the immediate detection of winding and rotor defects prior to disassembly and test. For smaller motors, this may mean that a repair versus replace decision can be made before the motor is disassembled, saving the repair shop and motor owner hours of lost time and repair costs.

If the motor passes these tests, it is disassembled and cleaned using solvent, hot soap and water, steam, or some other accepted method. If the stator has been cleaned with soap and water, it must be dried, before further testing, in an oven set for a temperature of around 196oF (90oC). If damage occurs to the insulation as a result of cleaning, or if the insulation appears to have minor defects, it may be dipped and baked in a Class F, or better, insulation varnish.

Once cleaned, the windings should have a second MCA test prior to an AC or DC hi-potential test performed at a voltage figured in Formula 1. The AC hi-pot is a pass/fail test, as if it arcs to ground, the insulation will be damaged beyond repair. The DC hi-pot is more forgiving, especially if the leakage can be monitored. Any sudden increase indicates that the insulation has failed. If it is below the calculated voltage value, when it fails, then the winding should be rewound.

Formula 1: Test Voltage

Vac = 0.65 * (2Em + 1000V)

Vdc = 0.65 * (2Em + 1000V) * 1.7


Em = the motor nameplate voltage


If the motor completes this test successfully, it should be Surge Comparison tested. The voltage value limit for this test is the same as that determined in Formula 1. In this test, however, wire insulation is being compared. This test is meant to detect shorts within the windings themselves. It is normally done by setting the surge tester to a value of Zero volts and bringing it up, slowly, to the calculated value. The tester sends a high frequency surge to the windings and the results are read on an oscilloscope comparing at least two of the windings at a time. Once properly set, any deviation in the scope waveforms indicate a defect. This test is considered a pass/fail test should a defect be detected, it will normally finish off the weakness. It must also be considered that the surge test will not detect broken turns, loose connections and may miss obvious shorts deeper than the first few turns of the windings. Therefore it should be coupled with the MCA test for greater accuracy.

There are no reasons why non-destructive tests, above and beyond these, may not be performed. A world -class quality repair shop will do whatever is necessary to ensure that no surprises occur during the motor repair process.

  1. Mechanical Tests

All of the mechanical fits on the motor must be tested using calibrated outside and inside micrometers. The critical areas which effect efficiency include the bearing journals and housings. If the fits are too loose, or tight, both the efficiency will be reduced and the bearing life will be reduced.

There are several ways to return bearing fits, which include:

  • Peening – The practice of punching or marring mechanical fits to create a tighter fit. This practice is not recommended for repair as it is “uncontrolled.”
  • Metalizing – Consists of a one or two part spray process which requires metal to be removed first. This process is susceptible to separation from the material to which it is attached in instances of non-symmetrical pressure, or when the surfaces have not been properly prepared. This practice should not be used for “world class” energy efficient motor repair.
  • Welding – Similar to metalizing, however, it creates a stronger metal to metal bond, when properly applied. If a repair requires adding metal, this is the preferred method.
  • Sleeving – The process of returning fits by machining and sleeving a motor shaft or housing. This is the recommended method of motor repair, as it is more controlled.
  • Refabrication – While expensive, this method is the best for machining severely worn motor parts, shafts in particular.  It is also highly recommended that motor bearings are replaced during each repair. They should also be replaced with the original class of bearing. Internal bearing fits and friction can have a large effect on motor efficiency. Fan replacement should also be considered, when the original fan has been damaged. The replacement fan should be original, as well. If a fan is replaced by a larger fan, or one with more fins, the motor efficiency will be reduced. If a fan is replaced by a smaller fan, or one with fewer fins, cooling will be reduced, reducing the life of the motor.

3. Coil Removal Practices

At this point, and for the purpose of this blog, it is assumed that the motor has failed at least one of the tests outlined above. The stator will have to be “stripped,” meaning that the copper windings will have to be removed, before re-insulating and rewinding the motor. It is “best practice” to perform a core test before and after the stator is stripped. The Wattage per pound losses should be recorded, and should be found not to increase.

In all the motor stripping practices, one end of the coil winding is removed. The length of the end-turns must be measured first and any connection and/or other information collected and recorded. Then one of the following methods is used for removing the remaining wire:

  • Direct Flame – A flame from a torch, or other source, is directed onto the core and winding. In some cases, the stator is physically placed in a bonfire! The temperature is uncontrolled and severe damage to the core will occur. The winding is reduced to ash, and the windings remove.
  • Chemical Stripping – The core is lowered into a chlorinated solvent bath and kept submerged until the varnish is dissolved enough for coil removal. Chemical stripping is ineffective in many cases, such as overloaded stators. The chlorinated solvent presents potential health, environmental, and disposal problems. In some cases, the solvent is not completely removed when the stator is rewound, and the solvent works against the new motor insulation.
  • Burnout – The stator is placed into a burnout oven that is set for a recommended temperature of 650oF (345oC). It is kept at this temperature until all of the varnish and insulating materials are turned to ash (8 hours, or more). If the temperature exceeds this, damage to the stator core and frame may result, reducing motor efficiency and mechanical reliability. Gasses, and other by-products, are exhausted through a “smoke stack” into the atmosphere.
  • Mechanical Stripping (Dreisilker / Thumm Method) – Using a heat source, such as gas jets, a distance away from the core, the back iron and insulation is warmed until the windings become soft and pliable (approximately 10oC above the insulation class of the varnish insulation). The coils and insulation are removed using a slow, steady hydraulic pull. Temperatures remain low, stripping times extremely fast (ie: 2.5 hours for a 350hp motor), and there are no airborne by-products nor disposal problems. Attempts at duplicating this process using pneumatic pulling methods have resulted in core laminations being pulled apart. Therefore, pneumatic machines, of this type should be avoided.
  • Mechanical Stripping (Water Blasting) – A high pressure stream of water is used to blast the coils out of the stator slots. This is a fast method of coil removal. Personal injury, due to high water pressure, and mechanical damage, can be avoided by experienced personnel and safety devices.
  • Mechanical Stripping (Hot Vapor Process Chemical Stripping) – A stator is submerged in a bath of non-chlorinated petroleum-based solvent at a temperature of 370oF (190oC) for a short period of time. It is then removed and the coils removed with high-pressure air. The solvent has an oily smell which must be masked, and is difficult to dispose of. Personal injury and mechanical damage can be avoided by experienced personnel and safety devices.

Once the windings have been removed, the stator may have to be cleaned. This may be done by steam cleaning and baking, bead or cob blasting, or low pressure air. In some cases, additional copper, that may have fused to the core at the time of motor failure, will have to be removed. This is done with a small air grinder or jeweler’s files.

The stator should then receive a loop test which is performed to check for “hot-spots” within the stator core caused by shorted laminations. If these are found, they may be removed by separating the effected laminations and insulating them, then pressing them back together. Other methods include a dip and bake before rewinding, or VPI’ing the stator core. In some cases, the core losses or hot-spots may be excessive causing the stator core to have to be re-stacked or the motor replaced.

4. Stator Winding

Common rewind practice dictates that the paper insulation inserted into the stator slots be of Class F insulating material, or better. The most common is Class H. The reason for this is to allow the motor insulation to survive any hot-spots which may have been missed during the loop or core loss tests. This also has the effect of potentially increasing the insulation life of the motor beyond the original design, and allowing some “forgiveness” if the original cause of insulation failure has not been corrected when the motor has been returned to service.

It is “best practice” to rewind the motor with the same wire size and type of coil winding method (lap or concentric). In some cases this is not possible. If the wire size must change, it must maintain the same cross-sectional area. A general rule of thumb is, for every three wire sizes smaller, two wires will be the same. For instance, if one number 15 wire is required, two number 18 wires may have to suffice. If the wire size is made smaller, the I2R losses will increase, decreasing motor efficiency and reliability, if it is made much larger, there is the chance of over-filling the stator slots, or increasing the motor’s inrush current. It is best to create a sample coil to ensure that the coil ends are the correct length and the coils will fit in the stator slots.

There are several coil winding methods:

  • Hand – Winding – Performed with a “tower-type” winding machine and mechanical counter. The winding technician must try to maintain correct tension and layering of the coils, or the coils will be difficult to lay in the stator slots. In the “worst-case,” there will be wires crossing which will increase the turn to turn potential in the wire, creating an area which may short under certain operating conditions. Improper tensioning of the coils may cause more wire per phase, changing the impedance balance of the motor windings.
  • Automatic Coil Winding Machines – Maintain constant tension and proper count of the coils. Still require a technician to observe operation, but still succeeds in reducing labor time.
  • Computerized Coil Winding Machines – The technician is free to perform other tasks while the machine winds the stator coils. Proper tension and turn count is maintained.

The coils are then inserted by hand or machine. It is important to include phase insulation and “in-betweens,” in order to avoid phase to phase or coil to coil shorts when the motor is returned to operation.

Once the coils have been inserted, the coil ends are insulated and connected. The stator connection must be the same as the original, and the coil ends crimped, silver-soldered, or braized. The lead wire must be of the correct size and type for the motor current and application. After this phase, the coil ends are tied down for mechanical strength. The ties should pass between each coil slot and tied. Care should be taken not to pull up the phase insulation.

5. Post Winding Tests

An insulation to ground test should be performed on the rewound stator of 500VDC, for motors rated under 600 Vac. The windings should show a resistance of better than 1000 Megohms (based upon experience).

A Hi-Potential test should be performed at a value calculated in Formula 2. Passing results and methods are outlined in the Initial Winding Tests. The Surge Comparison Test should be the same as in the Initial Winding Tests, except at the Formula 2 value. It should be noted that the surge test will act as a pass/fail and will not detect loose connections, broken conductors nor defects deeper than the first few turns of the coils conductors. Therefore, an MCA test is recommended along with the surge test for greater results.

Formula 2: Test Voltage

Vac = 2Em + 1000V


Vdc = (2Em + 1000) * 1.7

Em = the motor nameplate voltage

Additional tests include an Impedance test(MCA) and a Spin test. The impedance test is a comparison between all three phases. The difference should not be more than +/- 3%. The Spin test consists of placing 10% of the nameplate three-phase voltage across three of the stator lead wires. A current reading is taken and compared. Then a ball bearing or test rotor is inserted into the stator core. If the windings are correct, the bearing should rotate within the stator core, or the test rotor will operate in the same direction as it is brought around the inside of the stator core.

All test results should be recorded for future reference.

6. Varnish Insulation

The final step in the rewind process is to varnish the stator. The purpose of varnish is to increase the mechanical and electrical strength of the stator windings. As with the slot insulation, it is common practice to use Class F or H varnish on the stators. There are several basic methods for insulating rewound stators:

  • Dip and Bake – The stator is pre-heated then dipped into a tank full of insulating varnish. This is normally done a minimum of two times to ensure a full coat of varnish. Care must be taken as voids may be left within the stator coils which may collect moisture, or other contaminants. Additionally, all of the surfaces, including machined areas, are covered with varnish, which must be removed (and constitutes wasted varnish material). While the slots are receiving a reasonable amount of varnish, to allow for heat conduction, a blanket of varnish collects on the outer surfaces of the motor, reducing its ability to cool itself.
  • Trickle Varnishing – The stator is placed on a turntable and connected to three-phase power. This both serves as a heating source for the windings and an additional powered test (the coils should heat evenly). The stator is heated horizontally and monitored with an infra-red sensor. Once the windings have reached a pre-determined temperature, the turntable is tilted to 35 to 45 degrees and varnish is trickled on to the windings through several tubes. The varnish is drawn through the slots by gravity and capillary action creating a solid slot fill. The varnish also collects on the end turns. In considerably less time than two dips and bakes, the stator windings will have the equivalent of three dips and bakes (1 to 2.5 hours as opposed to 16 to 20 hours). There is no excessive varnish, decreasing cleaning time and varnish waste.
  • Vacuum Pressure Impregnation (VPI) – Due to expense, this process is not recommended for low voltage stators, but is a must for medium voltage, form wound cores. It consists of a voidless slot fill (as the trickle varnish method), but wastes varnish ( as does the dip and bake). The stator is warmed in an oven, then placed in a VPI tank. A vacuum is drawn within the tank, then varnish is flushed in from a holding tank.       A pressure is then applied to the tank forcing varnish into all existing voids. The stator must then be placed in a baking oven to cure the varnish.
  1. Rotor Tests

The rotor should be tested upon disassembly, using MCA, or during the repair evaluation phase using growler, die or single-phase testing. The rotor must be balanced with all rotating components mounted on the shaft and at least a half-key in any open keyways.

  1. Final Tests

Once the stator has been varnished and cleaned, noting that abrasives on the stator laminations may cause shorting between laminations, the motor is assembled. (In “world class” repair centers, the stator is retested before assembly.) An insulation to ground test is performed once the motor has been assembled, and should measure at least 1000 M-ohms. The electric motor is then tested at no load and all rated voltages for 30 minutes. The current and voltage is measured and recorded, if the motor had been tested during the disassembly phase of the repair, the final results are compared to the first. Also, the temperature of the stator is checked, and should remain cool to the touch, when operated at no load (also assuming the motor is not an “air-over” motor).

The measured current readings are compared, and, if found to be in excess of 5% of each other, the phases are rotated. For example: Phase A is rotated to the Phase B location, B to C, and C to A. If the unbalance remains the same and is found to follow the line leads, then the power supply is unbalanced, if the unbalanced current remains on the motor leads, then the rewind repair is suspect and the motor should be disassembled to have the stator retested and repaired.

Motor current should also not exceed the nameplate rating during a no-load test. The “rule of thumb” for two, four, and six pole motors is that the no-load current will be in the area of 25 to 50 percent of nameplate.

It is also recommended that either a vibration analysis or Electrical Signature Analysis (ESA) is performed under part load in order to detect any operating defects prior to shipment.

Once all the running tests are complete and acceptable, the motor is electrically suitable for operation. In a few cases, the customer may require additional tests.

  1. Conclusion

As shown, there is more to an electric motor repair than a good looking paint job. The type and quality of work required for returning a “world class,” “good as new” electric motor following a rewind repair is extensive. It is apparent that a motor repair customer must work closely with a motor repair center to ensure that the equipment, which is sent out for rewind repair, is handled in a manner which does not reduce efficiency nor reliability.

An end-user should have pre-qualified an electric motor repair shop to ensure that their equipment will be repaired to their expectations. This prequalification should include a review of capabilities, equipment, a recognized quality control program (ISO 9000 or EASA-Q recommended), and a method for handling warranties or concerns. The end-user should ensure that all billing, terms and conditions, and reporting is understood by both parties in advance. It is also recommended that the end-user has a method for contacting the motor repair center at any time.

To achieve this, a specification for motor repair, to include pre-qualification requirements, developed through a neutral entity for fairness, must be developed to ensure that the end user is receiving the best energy efficient and cost effective repair or repair versus replace decision possible.

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